In prior art electrostatic copying machines, a toner image is most often fixed to a sheet of paper using a heating roller. That is, a sheet of paper having a toner image formed thereon is passed through a pair of rollers in pressure contact, one or both of which are internally heated. Most advanced electrostatic copying machines adopt this fixing method using a heating roller because it has the advantages of heat efficiency and high speed operation over other fixing methods using an oven, for example.
Most of the currently used rolls for fixing toner images are silicone sponge rolls because they are heat resistant and can provide clear and soft images. However, the conventional silicone sponge rolls are insufficient in toner release so that the resulting images produce therewith are somewhat imprecise or indistinct. For promoting toner release, it has been a common practice to coat the silicone sponge roll on the surface with a liquid silicone rubber layer having toner releasability or to integrate a fluoride release layer with the roll surface. These methods undesirably complicate the overall manufacture process of silicone sponge rolls and add to the cost of fixing rolls.
The silicone sponge used for fixing rolls is required to be a foam body having a dense and uniform cellular structure. Azobisisobutyronitrile is used as a foaming agent in the prior art. Decomposition residues of this foaming agent are detrimental to a safe and hygienic environment and require long term heating for substantial decay.